14.1 14. Shapes of Molecules (text Ch 5 and 7)A. Molecular Geometry •Lewis structures provide us with the number and types of bonds around a central atom, as well as any NB electron pairs. They do not tell us the 3-D structure of the molecule. CHHHHCH4as drawn conveys no 3-D information(bonds appear like they are 90° apart)•The Valence Shell Electron Pair Repulsion Theory (VSEPR), developed in part by Ron Gillespie at McMaster in 1957, allows us to predict 3-D shape. This important Canadian innovation is found worldwide in any intro chem course. •VSEPR theory has four assumptions 1. Electrons, in pairs, are placed in the valence shell of the central atom 2. Both bonding and non-bonding (NB) pairs are included 3. Electron pairs repel each other Æmaximum separation. 4. NB pairs repel more strongly than bonding pairs, because the NB pairs are attracted to only one nucleus •To be able to use VSEPR theory to predict shapes, the molecule first needs to be drawn in its Lewis structure.
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